Electrolytic bath for use in electrodeposition of ferromagnetic compositions



ELECTROLYTEC BATH FOR USE IN ELETRO- DEPOSITIGN OF FERROMAGNETIC COMPOSI- TIONS Ignatius Tsu and Jerome S. Sallo, Dayton, Ohio, assignors to The National Cash Register Company, Dayton, Ohio, a corporation of Maryland No Drawing. Filed Apr. 2, WW, Ser. No. $33,585

11 laims. (Cl. 204-43) This invention relates generally to the manufacture of ferromagnetic memory elements such as bobbin and toroidal-shaped cores, and those commonly termed twistors and bit wires, and the like, which are adaptable for use as magnetic memory elements in present-day electronic computers and data processors. More specifically, the present invention relates to a new and improved electrolytic bath for use in the manufacture of such elements, which elements possess greatly improved magnetic and other characteristics than heretofore possible.

In most electronic computers and data processor applications, it is generally desirable that the magnetic memory elements be relatively small in size, require negligible expenditure of time and effort in order to be electrically connected in circuit, be physically sturdy and economical to manufacture utilizing mass production techniques, possess relatively high magnetic remanence and relatively low magnetic coercivity properties, be capable of being switched from one magnetically-stable state to another in a unit of time measured in microseconds or fractions thereof, and, additionally, possess substantially rectangular hysteresis characteristics, which results in a substantially high operational signal-to-noise ratio.

Various attempts have heretofore been made to produce such magnetic memory elements for information storage purposes by electrodepositing a relatively thin coating of rectangular-looped ferromagnetic material onto an electrically-conductive substrate. However, even though present-day electrodeposition techniques possess many highly desirable potentialities in the manufacture of such elements, to date there is yet to be economically produced thereby any commercially acceptable memory elements which possess the above-mentioned characteristics.

Consequently, the primary object of the present invention is to devise a new and improved aqueous electrolytic bath for utilization in the process of electrodeposition of a ferromagnetic coating onto an electrically conductive substrate whereby new and improved magnetic data storage elements are economically produced by mass production techniques and which elements possess the above-mentioned desirable characteristics.

In accordance with a preferred aspect of the present invention, there has been devised a new and improved aqueous electrolytic bath for use in the process of electrodeposition of a ferromagnetic coating onto an electrically conductive substrate, in which process the substrate is subjected as a cathode to electrolytic action in the bath. Such a bath includes as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to grams per liter, and a complexing agent, the bath having a pH in the range of 3.5 to 7.

More specifically, the plating bath preferably contains simple salts of iron and nickel in complexed form wherein iron is initially added as a ferric salt such as ferric chloride (FeCl -6H O) and nickel is preferably added as a simple nickel chloride salt (NiCl -6H O). From the standpoint of economic availability, the chlorides are preferred; however, any salt of iron or nickel may be used provided the anion does not cause precipitation in the overall system. Following the addition,

several iron and nickel species may appear as hydrated ions, metal chelates, and complexing and wetting agent complexes. The form in which nickel and iron exist in a given system depends upon many factors, such as the path pH, temperature, and constituent concentrations therein of nickel and iron ions, in addition to chelating, addition, and complexing agents. It is by varying these factors that a wide range of magnetic properties is obtained. The nickel and iron species in the bulk of the solution are not necessarily the same as those species occurring in the so-called double layer at the cathode. It is the latter species from which deposition actually occurs and which is of ultimate importance in determining the magnetic properties of the deposit. However, the species present in the bulk of the plating solution are instrumental in determining the nature and concentration of the species present in the cathode double layer.

Due to the fact that the hydrated oxides of iron begin to precipitate even in acid solutions, it is necessary to utilize a complexing agent to maintain the plating bath in solution. The preferred complexing agent for this purpose is ammonium citrate (NH HC H 0 However, any material which forms a complex with the iron ions of sufiicient solubility and stability is a suitable com plexing agent. For example, complexing agents which may be used with equal success are sodium citrate (Na C H O -2H O), and potassium citrate (K C H O glycolic acid (C H O aspartic acid (C H O N), and the sodium salt of ethylenediaminetetra-acetic acid etc., may be used, provided there is no electrolytic breakdown of the complex, for example, due to the occurrence of anodic oxidation during the plating process, which might form oxidation products at the anode, thus modifying the plating bath and, consequently, modifying the magnetic properties of the cathode deposit.

In the present invention, it is believed that the essential function of the complexing agent is to provide a soluble reservoir of iron and nickel ions from which are formed, through dynamic equilibria, the species from which the deposition actually occurs. The complexing agent must be of a concentration in the bath to supply the iron and nickel species rapidly enough through equilibria to provide a suitable concentration" of species for deposition, and yet, not rapidly enough to form an appreciable amount of other species whose solubility limits are exceeded to cause precipitation in the bath. Since the complexing agent partially determines the concentration of the reactive species present in the plating bath, the choice of the particular complexing agent has an effect on the composition and structure, and consequently the magnetic properties, of the cathode deposit, which influence may be modified by the other constituents of the plating system. In the preferred plating bath, it is desirable that a minimum of a molar ratio of 1 to 1 be maintained between the relative citrate and iron ion concentrations.

In prior plating baths, such as those disclosed in copending applications Serial No. 764,522, of Ignatius Tsu et al., filed October 1, 1958, and Serial No. 773,843, of Jerome S. Sallo et al., filed November 14, 1958, both of which applications being assigned to the present assignee of the instant application, it has been found that the various constituent concentrations within the baths are quite diflicult to control, and, consequently, the usable life of each bath is somewhat limited, and thus uniformity of the structural composition of the electrodeposit is quite difficult to maintain. One of the major reasons for such difiiculty of control is that ammonia (NH is depleted due to vaporization at the working temperature of the,

bath, and, consequently, it is necessary to continuously replenish the bath with ammonium hydroxide to maintain the pH thereof at the preferred alkalinity value. In order to alleviate the problem of bath stability, it has been found that it is necessary for the pH thereof to be in the acidity range in the order of from 3:5 to 7. It is well known that any acid or base may be used to control the PH; e s ium ydrox de (Na n y chloric acid (HCl) are preferred. The use of sodium hydroxide instead of ammonium hydroxide, as required in the previously mentioned baths, eliminates the problem of maintaining the pH through the use of a volatile base and thereby simplifies bath control.

'It has been discovered that by lowering the pH of the bath from the alkalinity range, as in the aforementioned co-pending applications, to a value in the acidity range of from 3.5 to 7, not only is there resulting therefrom a more stable bath as far as the major constituent concentrations are concerned, but, in addition, an entirely new and improved plating system is derived therefrom. Due to the fact that different degrees of ionization of citrate have been found to be a function of bath pH, the cationic ferric complexes in the bath therefore also vary with bath pH. For example, at a bath pH of 5.0, the percentages of iron and nickel in the deposit are approximately 38 and 62 percent, respectively. However, at a bath pH of 6.0, the percentages of iron and nickel in the deposit are approximately 18 and 82 percent, respectively. In the previously mentioned plating baths, nickel was deposited from the amine complex and also the citrate complex, the ratio of these complexes reaching the cathode determining the composition and structure of the magnetic deposit. However, in the present bath, deposition occurs primarily from the citrate complex, since the ammonia (NI-I concentration is very low even when ammonium hydroxide (NHrOI-I) is used as a base.

It is also preferred that a wetting agent, such as sodium lauryl sulfate ('NaC I-I SO be added to the plating bath to lower the surface tension of the electrolyte and to facilitate wetting of the cathode and thus permit a release of entrapped gas from the deposit and an improvement in the magnetic properties thereof.

Shown in the following chart is such a new and improved electrolytic bath having preferred constituent concentrations in accordance with the teachings of the present invention. It is to be noted that, in the upper half of the chart, is given the concentration of each compound in the actual bath measured in grams per liter of aqueous solution. -In the lower half of the chart is listed the concentration in grams per liter of aqueous solution of each constituent present in solution as contributed by each compound. I11 each instance, the minimum, optimum, and maximum concentrations for each compound constituent are given in tabular form. However, it is to be of course appreciated that the upper and lower concentration limits of each compound and constituent of the bath are not critical in that they specifically define limits above and below which is a definite zone of demarcation of all useful magnetic properties possessed by the cathode deposit.

Plating Bath Compounds Min. Opt. Max.

Ferric Chloride (FeCh-SHZOL 2 4 8 Nickel Chloride (NiGlz-GHzOL- t 15 20 40 Ammonium Citrate [(NHmHCBHBOr]. 40 Z 120 Ammonium Chloride NH4C1 30 s0 70 Sodium Lauryl Sulfate NaCqlfIzsSO; 0. 04 0.05 0.1 pH (Sodium Hydroxide Addition) 3. 5 6

Plating Bath Constituents Min Opt. Max

Ferric Ions 4 8 2 Nickel I0ns. 3 4. 9 10 filtrate Ions I 132 mmonium ons Sulfate Ions- .01 .015 .04

Even though the just-tabulated electrolytic bath finds great utility in a number of present-day electroplating processes, a preferred process will now be described which utilizes the novel bath of the present invention to fabricate an improved magnetic data storage device of the type shown and described in co-pending application Serial No. 791,695, of Ignatius Tsu, filed February 6, 1959, such a helical flux device having a substantially high anisotropy such that an easy direction of magnetization exists in the deposit in a helical direction without the necessity of a continuous torsional strain being applied to the ends of the device as discussed in co-pending application Serial No. 696,987, of John R. Anderson et al., filed November 18, 1957, and assigned to the present assignee.

After the just-described electrolytic bath has been prepared, having preferred constituent concentrations in accordance with the just-tabulated bath, the pH thereof is adjusted to a value in the range of 3.5 to 7, preferably 6, by the addition thereto of a suitable amount of sodium hydroxide (NaOI-I). Even though the bath may be successfully operated at ordinary room temperature, the temperature thereof is preferably adjusted to approximately degrees centigrade. Thereafter, the bath is introduced into a conventional rubber-lined steel-plated tank or into an equivalent inert container.

The substrate onto which the electrodeposit is to be formed may be composed of a variety of electricallyconductive materials such as alloys of copper, silver, aluminum, brass, bronze, etc., or any of the known electr-ically-conductive elements. The physical shape or surface configuration of the substrate is not critical and may be tubular, toroidal, planar, rod, or ribbon-shaped. In fact, the substrate may even be an extremely thin electrically-conductive film which is mechanically supported by an insulating material such as glass, plastic, ceramic, or the like. However, it is preferred that the substrate be in the form of a hollow Phosphor-bronze wire-like tubing having an outside diameter of approximately 15 mils and an inner diameter of approximately 8 mils.

It is also preferred that the tubular substrate be first provided with a plurality of substantially V-shaped and equally-spaced grooves which are helically disposed about the periphery of the substrate, the grooves having a depth of approximately 25 micro-inches, a spacing of approximately 1 micro-inch, and a pitch of approximately 60 degrees.

After the grooves have been formed in the substrate, the substrate is then cleaned in a conventional manner before plating through the use of any of the well-known alkaline-acid-Water methods as used by present-day electroplating industries. As in the previously-described twistor and bit wire types of magnetic data storage devices, it is desirable to secure a relatively thin ferr magnetic deposit on the substrate which has a thickness in the order of one ten-thousandth of an inch so as to maintain eddy current losses therein at a minimum and yet be thick enough to insure adequate readout voltage during operation of the device as a memory element. Consequently, it is necessary that the substrate be exposed as a cathode to electrolytic action in the bath for only a short period of time, preferably in the order of one minute, depending upon, of course, the particular value of cathode current density chosen to be used in the plating process. To accomplish this, the process is made a continuous one whereby the wire-like substrate is moved through the bath at a constant speed by any well-known means, with electrical contact at all times being maintained with the substrate to supply the plating current thereto. It is also preferred that the substrate be centrally encompassed at all times while in the bath by a helical-shaped anode having a coil diameter of approximately one inch and composed of an electrically-conductive wire of approximately 50 mils in diameter.

The choice of anode material may not be arbitrarily made. However, platinum and iron-nickel anodes may successfully be used, provided sludge formations originating at the iron-nickel anode do not enter the bath solution. Due to the fact that the remaining compound constituents in the bath are depleted during the plating process when platinum anodes are used, it is also generally desirable to continually add solutions of the bath compounds to the plating bath to replenish the ionic content thereof as contributed by each of the compounds. A suitable iron-nickel anode may be used to replenish the depleted salts.

The current density involved in the deposition process is not critical and may range, for example, from 250 to 1,000 (preferably 500) amperes per square foot of substrate surface area exposed in the bath. The current density primarily determines the rate of deposition of the metallic ions onto the cathode and also affects the rate of diffusion into the cathode film which influences the amount of depositing species which must be in equilibrium with the reservoir complexes. Consequently, the bath constituents, as contributed by each of the compounds therein, and the current density of the process must be compatible, and, therefore, the current density may not be arbitrarily chosen. As the current density is one of the prime factors which determine the composition and structure of the deposit, it is generally necessary to modify the plating system to permit the use of a specific current density.

On emergence from the plating bath, the ferromagnetic element is rinsed and dried and is then ready to be incorporated into the electrical circuitry of present-day electronic computers and data processors and operated as a coincident current memory element in the following manner: Due to the fact that the easy direction of magnetization of the ferromagnetic deposit is in the same direction as the substrate grooves, as in co-pending application Serial No. 791,695, and as the deposit has also been found to possess a substantially high positive and negative magnetic remanence and a substantially rectangular hysteresis characteristic, selected length portions of the coating are allowed to attain one or the other of two stable conditions, respectively characterized by :a positive or negative magnetic remanence. Thus, a magnetic field of :H oersteds switches the length portion of the element from one magnetic state to another, Whereas a field of iH/Z oersteds produces only negligible changes in the magnetic remanence of the coating. Consequently,

- a plurality of similar coils may be separately wound about the coated substrate and positioned in a spaced sidebyaside relationship with respect to one anothen to encompass and thereby define a corresponding plurality of helical-path length portions of ferromagnetic material. Storage of binary information in a selected length portion of the coating is accomplished by sending a current impulse of half-select magnitude into the conductive wire of the common core and simultaneously sending a current impulse of half-select magnitude into the selected coil in such directions that the vector summation of the magnetic fields produced by both of the coincident half-select currents is at least equal in magnitude to :H oersteds and is additionally oriented in the same direction as the easy magnetization of the coating.

During reading of a selected length portion of the coating, either the core or the corresponding coil is pulsed with a current impulse of full-select magnitude to individually develop a magnetic field of at least iH oersteds in the opposite direction from the magnetic field developed during storage of the function. In response to the read impulse, an electrical signal is or is not available across the ends of the core or the corresponding coil, depending upon which one was pulsed, according to whether the binary information (1) or (0) had previously been established in that length portion of the coating 6 encompassed by that particular coil, as represented by its positive or negative magnetic remanence.

Listed below in chart form. are the electrical operational characteristics of the magnetic data storage devices fabricated from the foregoing bath. In the chart, (Ir) represents the half-select current impulse applied across the ends of the ferromagnetic element during storage of the function; (Is) represents the half-select current impulse applied across the ends of the coil during storage of the function, the coil being circumferentially wound about the ferromagnetic element in the same manner and for the same purposes as heretofore described; (Is') represents the full-select current impulse applied across the ends of the coil during reading of the function; (uVl) represents the instantaneous readout potential appearing across the ends of the ferromagnetic element indicative of any reversal of the magnetic remanence of the element, i.e., due to pervious binary 1 storage; and (dV represents the undesirable operational noise potential of the storage element.

It has been observed that the ferromagnetic coating deposited from the present bath actually has no preferred direction of magnetization, per se, when deposited on a smooth-surfaces substrate. However, when deposited on a substrate having a plurality of microgrooves formed on the peripheral surface thereof, as just described, the mag netic device has been found to possess far superior mag netic characteristics to any of those described in the previously-referred-to co-pending applications. Although the exact phenomena are not fully understood, it is believed that the greatly improved magnetic characteristics are predominantly attributable to a more readily crystal alignment characteristic of such a deposit with respect to the substrate grooves.

From the foregoing, it is now evident that in accordance with the present invention, there has been devised a new and improved aqueous electrolytic bath for use in presentday electrodeposition processes whereby new and improved magnetic data storage devices may be econon1- ically fabricated by mass production techniques, possess greatly improved magnetic and other characteristics than heretofore possible, and are readily adaptable for incorporation in present-day electronic computers and data processors. 7

While particular elements of the invention have been shown and described, it will be obvious to those skilled in the art of electrodeposition of ferromagnetic materials that changes and modifications may be made without departing from the invention in its broadest aspects, and, therefore, the aim of the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath having a pH in the range of 3.5 to 7 and including as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in a range of 3 to grams per liter, and a complexing agent capable of forming solu ble iron and nickel complexes and being of sufficient concentration to prevent precipitation of said iron and nickel ions.

2. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath having a pH in the range of 3.5 to 7 and including as essential constituents ferric ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in a range of 3 to 10 grams per liter, and a complexing agent capable of forming soluble ferric and nickel complexes and being of sufficient concentration to prevent precipitation of said ferric and nickel ions.

3. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularlyspaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath having a pH in the range of 3.5 to 7 and including as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in a range of 3 to 10 grams per liter, and a complexing agent consisting of citrate ions in a concentration in the range of 34 to 100 grams per liter.

4. A process for fabricating magnetic computing devices comprising the steps ofi providing an electrically conductive substrate having a plurality of substantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath having a pH in the range of 3.5 to 7 and including as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, ammonium ions in a concentration in the range of 10 to 24 grams .per liter, and a complexing agent capable of forming soluble iron and nickel complexes and being of sufficient concentration to prevent precipitation of said iron and nickel ions.

5. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath containing ferric chloride and nickel chloride and including as essential constituents ferric ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, and a complexing agent capable of forming soluble ferric and nickel complexes and being of sufficient concentration to prevent precipitation of said ferric and nickel ions, said bath having a pH in the range of 3.5 to 7.

6. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularlysspaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath containing ferric chloride and nickel chloride and including as essential constituents' ferric ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, and a complexing agent consisting of citrate ions in a concentration in the range of 34 to grams per liter, said bath having a pH in the range of 3.5 to 7.

7. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality ofsubstantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath including as essential constituents ferric ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, ammonium ions in a concentration in the range of 10 to 24 grams per liter, and a complexing agent consisting of citrate ions in a concentration in the range of 34 to 100 grams per liter, said bath having pH in the range of 3.5 to 7 by the addition of sodium hydroxide.

8. A process for fabricating magnetic computing devices comprising the steps of: providing an electrically conductive substrate having a plurality of substantially regularly-spaced grooves formed in the surface thereof; and depositing a ferromagnetic coating onto said substrate surface by subjecting said substrate as a cathode to electrolytic action in an aqueous bath having a pH in the range of 3.5 to 7 and including as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, a complexing agent capable of forming soluble iron and nickel complexes and being of sufficient concentration to prevent precipitation of said iron and nickel ions, and a Wetting agent.

9. An aqueous electrolytic bath for use in the process of deposition of a, ferromagnetic coating on an electrically conductive substrate in which process said substrate is subjected as a cathode to electrolytic action in said bath, said bath having a pH in the range of 3.5 to 7 and including as essential constituents iron ions in a concentration in the range of .4 to 2 grams per liter, nickel ions in a concentration in the range of 3 to 10 grams per liter, and a complexing agent capable of forming soluble iron and nickel complexes and being of sufiicient concentration to prevent precipitation of said iron and nickel ions.

10. An aqueous electrolytic bath for use in the process of deposition of a ferromagnetic coating on an electrically conductive substrate in which process said substrate is subjected as a cathode to electrolytic action in said bath, said bath having a pH of approximately 6 and including as essential constituents ferric ions in a concentration in the order of .8 gram per liter, nickel ions in a concentration in the order of 4.9 grams per liter, and a complexing agent capable of forming soluble ferric and nickel complexes and being of sufiicient concentration to prevent precipitation of said ferric and nickel ions.

11. An aqueous electrolytic bath for use in the process of deposition of a ferromagnetic coating on an electrically conductive substrate in which .process said substrate is subjected as a cathode to electrolytic action in said bath, said bath containing ferric chloride, and nickel chloride, and including as essential constituents ferric ions in a concentration in the order of .8 gram per liter, nickel ions in a concentration in the order of 4.9 grams per liter, a complexing agent consisting of citrate ions in a concentration in the order of 5 8 grams per liter, and a wetting agent comprising sodium lauryl sulfate in a concentration in the order of .05 gram per liter, said bath having a pH of approximately 6.

(References on following page) References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Ernst et al.: Journal Electrochemical 800., vol. 102

Burns et a1. Dec. 22, 1931 (August 1955), pp. 461-469. Man'nis May 9, 1950 Haynes: Elements of Magnetic Tape Recording, 1957, Holt et a1. June 3, 1952 5 38,

FOREIGN PATENTS Australia Oct. 22, 1929 

1. A PROCESS FOR FABRICATING MAGNETIC COMPUTING DEVICES COMPRISING THE STEPS OF: PROVIDING AN ELECTRICALLY CONDUCTIVE SUBSTRATE HAVING A PLURALITY OF SUBSTANTIALLY REGULARLY-SPACED GROOVES FORMED IN THE SURFACE THEREOF AND DEPOSITING A FERROMAGNETIC COATING ONTO SAID SUBSTRATE SURFACE BY SUBJECTING SAID SUBSTRATE AS A CATHODE TO ELECTROLYTIC ACTION IN AN AQUEOUS BATH HAVING A PH IN THE RANGE OF 3.5 TO 7 AND INCLUDING AS ESSENTIAL CONSTITUENTS IRON IONS IN A CONCENTRATION IN THE RANGE OF .4 TO 2 GRAMS PER LITER, NICKEL IONS IN A CONCENTRATION IN A RANGE OF 3 TO 10 GRAMS PER LITER, AND A COMPLEXING AGENT CAPABLE OF FORMING SOLUBLE IRON AND NICKEL COMPLEXES AND BEING OF SUFFICIENT CONCENTRATION TO PREVENT PRECIPITATION OF SAID IRON AND NICKEL IONS. 